linux-kernel - Re: [PATCH 1/1] NBD: fix I/O hang on disconnected nbds

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Message-Id: <20090210133537.23b1779a.akpm@linux-foundation.org>
Date:	Tue, 10 Feb 2009 13:35:37 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Paul Clements <paul.clements@...eleye.com>
Cc:	linux-kernel@...r.kernel.org, jnelson-kernel-bugzilla@...poni.net,
	Pavel Machek <pavel@....cz>, stable@...nel.org
Subject: Re: [PATCH 1/1] NBD: fix I/O hang on disconnected nbds

On Mon, 09 Feb 2009 13:21:51 -0500
Paul Clements <paul.clements@...eleye.com> wrote:

> This patch fixes a problem that causes I/O to a disconnected
> (or partially initialized) nbd device to hang indefinitely. To reproduce:
> 
> # ioctl NBD_SET_SIZE_BLOCKS /dev/nbd23 514048
> # dd if=/dev/nbd23 of=/dev/null bs=4096 count=1
> 
> ...hangs...
> 
> This can also occur when an nbd device loses its nbd-client/server
> connection. Although we clear the queue of any outstanding I/Os after 
> the client/server connection fails, any additional I/Os that get queued 
> later will hang.
> 
> This bug may also be the problem reported in this bug report:
> http://bugzilla.kernel.org/show_bug.cgi?id=12277
> 
> Testing would need to be performed to determine if the two issues are 
> the same.
> 
> This problem was introduced by the new request handling thread code
> ("NBD: allow nbd to be used locally", 3/2008), which entered into 
> mainline around 2.6.25.

Seems that the patch applies to 2.6.26, but not to 2.6.25.

> 
> The fix, which is fairly simple, is to restore the check for lo->sock
> being NULL in do_nbd_request. This causes I/O to an uninitialized nbd to
> immediately fail with an I/O error, as it did prior to the introduction 
> of this bug.

I marked this as needing backporting into 2.6.26.x, 2.6.27.x and
2.6.28.x.  OK?

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